Pennsylvania's Newest Noxious Weeds

In recent months the Pennsylvania Department of Agriculture has added several plants to its Noxious Weed List, meaning these plants will soon be banned from sale in Pennsylvania and will not be allowed to be transported or propagated anywhere in the state. (The current Noxious Weed List can be found on the PA Department of Agriculture website). For plants that are available in the horticultural trade, the enforcement of these bans is phased in over a period of time to allow nurseries to sell off current inventory. People who already have these plants on their property are not required to remove the plants, but it is highly encouraged due to the harmful impact these plants have on our native ecosystems. This article will briefly discuss each of the most recent Noxious Weed additions, along with general control methods and suggested native plants that could be used as alternatives for landscaping.

Callery pear (Pyrus calleryana)

The callery pear, also known as Bradford pear, has been one of the most popular landscape trees in North America since its introduction from Asia in the early 1900’s. This tree grows up to 40 feet tall and has a somewhat distinctive growth form resembling a teardrop or pyramid shape. It has shiny leaves that grow alternately on the stems, but probably the most recognizable feature of the callery pear tree is its showy display of white flowers that appear in early spring, before leaf-out. These flowers may look nice, but they produce a foul odor. The callery pear is mainly spread by birds and other animals that eat the small hard fruits and disperse the seed in their droppings. This tree can also spread vegetatively by root sprouts. Callery pear can tolerate some shade but thrives in full sun and most often invades open meadows, roadsides, and forest edges.

To rid your property of callery pear, mechanical methods like hand-pulling, digging, or using an uprooting tool (such as a Weed Wrench®) can be effective for seedlings and small trees. For larger trees or larger infestations, herbicides will be the most effective tool. Stem treatments including cut-stump, hack-and-squirt, and basal bark can be used anytime of the year. Foliar applications can be made when the tree is fully leafed and green, generally June through October. Foliar treatments are best used on trees less than 10 feet tall. To make foliar spraying easier and less risky to nearby plants, you can cut the tree down at the base and then treat the green resprouts from the stump the following year. For more detailed control information, see "Callery Pear."

Some attractive native trees that could be used for landscaping in place of callery pear include Allegheny serviceberry (Amelanchier canadensis), flowering dogwood (Cornus florida), alternate-leaf dogwood (Cornus alternifolia), hawthorns (Crataegus spp.), Eastern redbud (Cercis canadensis), white fringetree (Chionanthus virginicus), and sourwood (Oxydendrum arboreum).

Japanese barberry (Berberis thunbergii)

Many people are probably familiar with this highly popular landscaping shrub, known for its dense, compact growth habit (which can make a useful hedge), attractive fall color, and varieties that range in leaf color from green to red to dark purple. Introduced to the United States in 1875, Japanese barberry was long used as a hardy ornamental and ‘living fence’ plant for containing livestock. This shrub has distinctive small oval or ‘spoon-shaped’ leaves, sharp singular spines along the stems, and bright red berries that ripen in the fall and persist on the plant through winter. These berries are attractive to birds and small animals which spread the seeds through their droppings. Japanese barberry also will root from drooping stems that touch the ground and will gradually expand in this manner. This hardy plant will grow in most sites, from dry to wet soil and full sun to deep shade. It is highly invasive in forests and other natural areas, and research has associated barberry with higher densities of deer ticks, because the dense growth form of the plant provides a microclimate within the canopy that is favorable to tick development and survival. Additionally, the dense and thorny nature of the plant is attractive to small rodents, including the white-footed mouse, which is the main source of Lyme disease.

Now that you’re appropriately freaked out, let’s discuss ways to get rid of barberry. As with most plants, small shrubs can be hand-pulled, dug, or wrenched from the soil. Be sure to wear thick gloves and long sleeves to protect against the sharp spines. The shrubs can be cut close to the base and the freshly cut surfaces treated with herbicide, or a basal bark treatment to the intact stems can be effective. Spraying the foliage is also effective from June through September. For more details about controlling barberry, see "Japanese Barberry."

Some native alternatives to Japanese barberry include coralberry (Symphoricarpos orbiculatus), inkberry holly (Ilex glabra), winterberry holly (Ilex verticillata), Virginia sweetspire (Itea virginiana), and northern bayberry (Myrica pensylvanica) among others. For those who like the fall color of Japanese barberry, the native chokeberries (Aronia spp.) are known for beautiful red shades of fall foliage. Also, although it doesn’t have red foliage, red-osier dogwood (Cornus sericea) is known for its bright red stems that provide winter interest to a garden.

Ravenna grass (Saccharum ravennae; or Tripidium ravennae)

Also sometimes called hardy pampas grass or plume grass, Ravenna grass is native to southern Europe and northern Africa. In the landscaping industry, this large grass has been prized for its stature and dense growth habit, which can add some privacy when planted along the edge of a yard, as well as its showy seed heads. Ravenna grass forms large, dense basal clumps (tussocks) of grayish-green narrow leaves (0.5 to 1 inch wide) that can grow to 4 or 5 feet long. The leaves have a white midvein on the undersides and dense hairs at the bases of the blades. The flower stems of Ravenna grass can reach 13 feet tall and end in large feathery plumes. These seed heads produce many tiny seeds that are spread by wind and water. This invasive grass prefers moist sites, like wetlands and riparian corridors, but can also invade harsh dry sites like gravel banks and roadsides. Unlike many other invasive plants, Ravenna grass is able to become established in new sites without the aid of disturbance to existing soils and vegetation. This ability makes Ravenna grass especially threatening to native plant communities and other natural areas.

Control methods for Ravenna grass are still being studied due to the relatively recent nature of this threat. The first step is to remove and bag all seed heads to prevent further spread. The rest of the plant can then be dug up and destroyed or bagged and disposed of in the trash. Remove as many roots as you can when digging up the plant and keep an eye out for resprouts. For chemical control of Ravenna grass the University of California, Davis reports that spraying the leaves with glyphosate during the summer months is effective.

Ravenna grass may be confused with several other exotic grasses including pampas grass (Cortaderia selloana), jubata grass (Cortaderia jubata), and Japanese/Chinese silvergrass (Miscanthus sinensis). The pampas grass and silvergrass are also both widely available and popular in the nursery trade, while the jubata grass has limited availability. While these grasses are not listed as Noxious weeds in PA, they should still be avoided as they all have been shown to have invasive tendencies in the U.S. and are considered invasive weeds by various ecological organizations. USDA Weed Risk Assessments for pampas grass and jubata grass determined both species to have a high risk of becoming invasive in large portions of the United States (a federal risk assessment has not yet been completed for Japanese silvergrass).

From top to bottom: Jubata grass (Cortaderia jubata), Japanese silvergrass (Miscanthus sinensis), and pampas grass (Cortaderia selloana) should also be avoided as ornamentals as they all have the potential to become invasive. Credit (top to bottom): John M. Randall, The Nature Conservancy, Bugwood.org; Chris Evans, University of Illinois, Bugwood.org; and John Ruter, University of Georgia, Bugwood.org

While there are no native grasses that look similar to Ravenna grass, the native eastern gamagrass (Tripsacum dactyloides), pink muhly grass (Muhlenbergia capillaris), switchgrass (Panicum virgatum), big bluestem (Andropogon gerardii), Indiangrass (Sorghastrum nutans), and prairie cordgrass (Spartina pectinata) are all attractive plants that can add visual appeal to a home landscape.

Glossy buckthorn (Frangula alnus or Rhamnus frangula) and Common buckthorn (Rhamnus cathartica)

Of these two invasive buckthorns, glossy buckthorn is still cultivated in the horticultural industry and is a more aggressive invader. Native to Eurasia, glossy buckthorn was introduced to the U.S. in the mid-1800s and was promoted as an ornamental and for conservation plantings. This plant is also commercially available as several cultivars including ‘Rhamnus Fine Line’ or ‘Fine Line Buckthorn’; ‘Columnaris’ or ‘Tallhedge’; and ‘Asplenifolia’ or ‘Cutleaf Buckthorn’.

Glossy buckthorn grows as a large shrub or small tree, up to 20 feet tall, often with multiple stems. The oval leaves are shiny and leathery with curved veins and smooth edges. The fruits are small drupes that change from green to red to dark purple/black when ripe. Fruiting occurs all season and often there will be multiple stages of ripeness on one tree at the same time. These fruits are eaten and spread by birds, and the seeds germinate easily, leading to extensive infestations in natural areas. Glossy buckthorn thrives in moist/wet sites, including wetlands, lakeshores, and riparian areas, but it will also invade drier sites. It is also tolerant of deep shade.

In natural areas, especially wetlands, glossy buckthorn often forms dense thickets that are difficult to control. Credit (both): Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Again, manual removal can be effective for low-density seedlings and small plants if all roots are also removed. However, glossy buckthorn tends to form dense colonies that can only be controlled with herbicides. As with the other woody plants in this list, stem treatments are effective year-round, while foliar treatments to plants less than 10 feet tall are effective during the summer months and into fall, as long as the leaves are green. When removing glossy buckthorn, contain any fruits you find to prevent spreading the plant more. Remember, if the plant is growing in or adjacent to an aquatic site, such as a wetland, lakeshore, or streambank, you should only use herbicides that are specifically labeled for use in aquatic sites. If you have a significant infestation on your property, it may be best to consult a professional.

Be aware that glossy buckthorn can be confused with native alder species, which also tend to grow in wet sites. Alders have leaves with toothed edges and their fruit resembles tiny pinecones, rather than berries. Do not take control measures until you positively identify glossy buckthorn.

The leaves of common buckthorn. Credit (top to bottom): Paul Wray, Iowa State University, Bugwood.org; Richard Webb, Bugwood.org

Common buckthorn, the other invasive, tends to grow in drier sites than glossy buckthorn and is not as shade tolerant. It is usually found in old fields, fencerows, forest edges, etc. Common buckthorn produces very similar fruit to glossy buckthorn, and also has oval leaves. However, common buckthorn leaves have finely toothed edges and the veins curve toward the tip of the leaf rather than to the sides. This plant is no longer available in the North American horticultural industry, but plenty of them still exist from decades of planting across the United States and Canada. Common buckthorn can be controlled using the same methods as glossy buckthorn, but it should be noted that common buckthorn is more difficult to control with foliar herbicide applications. More information about buckthorns can be found at "Buckthorn."

Native alternatives to these invasive buckthorns include Allegheny serviceberry (Amelanchier canadensis), highbush cranberry (the native variety Viburnum opulus var. Americanum), Carolina buckthorn (Rhamnus caroliniana), lance-leaved buckthorn (Rhamnus lanceolata), alder-leaved buckthorn (Rhamnus alnifolia), wild plum (Prunus americana), grey dogwood (Cornus racemosa), silky dogwood (Cornus amomum), red-osier dogwood (Cornus sericea), and flowering dogwood (Cornus florida).

Japanese stiltgrass (Microstegium vimineum) and Garlic mustard (Alliaria petiolata)

(Top) Japanese stiltgrass plants, with distinctive silvery stripe down leaf centers. (Bottom) Japanese stiltgrass infestation in a forest. Credit (both): Dave Jackson, Penn State

While not found in the nursery trade, Japanese stiltgrass and garlic mustard were also both added to the Noxious Weed List in Fall of 2021. These are both aggressive invaders of natural areas and property owners are strongly encouraged to control infestations of these plants on their land. Japanese stiltgrass is especially prone to invading large areas. It is not difficult to control as a plant, but infestations are difficult to manage due to the persistence of the seedbank, so consulting a professional vegetation manager is recommended for large infestations. More information about these plants can be found at "Japanese Stiltgrass" and "Garlic Mustard."

Summary

The addition of these and other invasive plants to the PA Noxious Weed list may seem inconvenient or frustrating to people who want them in their home landscapes, but banning the use of harmful plants truly is for the greater good. Choosing native plants for landscaping has numerous benefits for property owners and for our native ecosystems. For example, native plants often require less water or fertilizer because they are adapted to the region. They also provide much-needed food and shelter for our native insects and wildlife, while most exotic plants are of little value to native wildlife. People are highly encouraged to use native plants for landscaping as much as possible, and any non-native plants should be non-invasive species or varieties. This is a simple way that homeowners can help reduce the threat and spread of harmful invasive plants. More information about landscaping with native plants can be found at:  

• Pennsylvania Department of Conservation and Natural Resources "Landscaping With Native Plants" website.
• Penn State Master Gardener Program's "Native Replacement for Invasive Plants" chart.